Blood for transfusions is typically stored at approximately 4° C. Prior to patient transfusions, it is preferable that the blood be warmed to a temperature substantially the same as that of the living body, approximately 37° C. Failure to warm the blood prior to transfusion, may result in the patient's core temperature dropping to dangerous levels potentially resulting in hypothermia.
There currently exists a number of portable blood warming devices used in hospitals. These are typically large and cumbersome devices that draw power from a mains supply. In some devices the blood is fed into a spiral tube, which is immersed in a water bath whose temperature is maintained at a set temperature. Other known devices use a heated jacket, which is wrapped around a blood bag, and draw power from the mains supply.
These types of devices, whilst satisfactory in hospital or clinical situations where a constant source of power is available, are not convenient to use in situations that are remote from reliable power sources or in remote locations where it is not practical to carry such large and cumbersome equipment, such as when a patient is moved between wards or geographical locations.
Portable devices for warming blood are also known, such as those devices sold under the THERMAL ANGEL and HOTIV trade marks. Both of these existing devices rely on batteries to provide the source for heating the infusion or transfusion fluid. There are however a number of problems with batteries including that they are heavy, bulky and require replacement or recharging at regular intervals. Given that heating equipment places substantial load on the batteries, it is not possible for these batteries to be small. Additionally, the maintenance issues associated with batteries are such that, for example, if the portable blood warmer is used in an emergency vehicle then it is necessary that regular checks are made on the condition of the battery or indeed that spare batteries are available such that there is always a source of power available. Furthermore, the requirement of batteries in order to operate the blood warmer dramatically increases the overall weight of the apparatus and therefore if rescue workers are required to trek into a remote location this then places an additional burden on them and may complicate matters if the unit fails to operate due to problems associated with the power source. The warming devices can also be used to heat infusion fluids, such as pharmaceuticals and saline fluids.
Portable devices for warming transfusion and infusion fluids using chemical processes to generate the required heat have also been attempted. Disclosures of such attempts can be found in patent documents, U.S. Pat. No. 5,042,455, U.S. Pat. No. 4,934,336, WO2003059414, WO2008017456, and by the present inventors' earlier application, WO2006056015.
One potential problem of using chemical reaction described in the prior art to generate the required heat, is the challenge of controlling and regulating the heat generated by the chemical reaction, which otherwise has the potential to damage the blood or infusion fluids if the temperature is raised too high. To overcome this problem, the inventors have experimented with using a chemical reaction that releases latent heat, upon solidification, at approximately 42° C., which is about the maximum temperature blood can tolerate before damage may occur. Results of an experiment undertaken by the inventors can be found in the disclosure, McEwen, M. P. and D. Roxby, “Can latent heat safely warm blood?—in vitro testing of a portable prototype blood warmer.” BMC Emerg Med, 2007. 7: p. 8.
The inventors have used crystals that react with a chemical liquid to produce the reaction that releases the required latent heat. However a difficulty in using these types of chemical reagents in currently available devices is that the flow of chemical liquid and crystals through the reaction chamber may be too slow. In addition the flow is further hampered by the crystallisation of the reagents that may result in blockages occurring within the reaction chamber. If this occurs the device may not be ideal in an emergency situation where the biological fluid warmer should preferably be fast acting, otherwise particularly in emergency situations the health of the patient may be compromised.
It is thus desirable to have a device for warming blood, or other fluids, that is portable, lightweight, disposable, reliable, economical in cost and manufacture, and that overcomes, or at least substantially ameliorates, the disadvantages and shortcomings of the prior art, or at least provides the public with a choice.
It should be appreciated that any discussion of the prior art throughout the specification is included solely for the purpose of providing a context for the present invention and should in no way be considered as an admission that such prior art was widely known or formed part of the common general knowledge in the field as it existed before the priority date of the application.